The art comprises methods for detecting possible defects in patterned wafers. A summary of the state of the art concerning such detection is included in U.S. Pat. No. 5,699,447, and is incorporated herein by reference. In said patent, an inspection apparatus is described which comprises a table for receiving a wafer to be inspected, a source of a laser beam, which beam scans the surface of the wafer, and a plurality of light collectors for collecting the light scattered from the wafer and transmitting the scattered light to a plurality of detectors. The output of the detectors is fed to a processor, which produces information indicating locations on the wafer in which the presence of a defect is suspected.
With optical scanning, it is possible to detect defects that are on top of the upper layer of the inspected wafer, or under transparent layers. As design rules progress, higher importance is given to detecting smaller defects. The general approach in the industry is to increase the imaging resolution by, for example, reducing the wavelength of the light source or replacing light with other radiation source.
However, when the upper scanned layer is comprised of high aspect ratio (HAR) structures, the defect detection becomes problematic. A typical defect that is difficult to detect, if impossible, is illustrated in FIG. 1. Numerals 10, 11 and 12 indicate respectively the metal layer, the dielectric layer, and the contact holes or "vias" in layer 11. The vias are formed in the etch process. When a contact hole or via 12 is filled with another metal to form the electrical connection between two metal layers, this connection should be well closed mechanically and electrically. In FIG. 1 it is seen that, after the etch process two contact or vias are completely obstructed or closed by non-conductive matter 13 and 14, and another via is partially obstructed or closed by non-conductive matter 15. The non-conductive matter may be polymer or residue dielectric material that has not been successfully etched (removed) or particles that have deposited on the wafer in the course of its processing. Such defects are called partially opened contacts/vias. Moreover, sometimes it happens that the process failed to create the contact holes entirely, e.g., due to problems with the mask or the stepper. Such a defect is called a "missing" contact hole.
Defects, such as partials (partially opened contacts/vias) and missing contact holes are crucial for the final yield of the FAB, because such defects are generally "killer" defects, i.e., they make the dies inoperative. With the current technologies of optical scanning, it is difficult to detect defects on the bottom of the vias or contacts, because of the following reason. The aspect ratio of contacts and vias, viz. the ratio of their height to their diameter, with today's technology is in the order of 5:1 to 10:1. The vias or contact diameter is about 0.25 microns and is going to decrease with the development of the technology. The wavelength of the light used for inspection illumination with today's technology is about 0.5 microns. Under such conditions, the light cannot reach the bottom of the vias or contacts, and thus, the defects there are not seen. The main reason is that the structure of the hole, having a higher dielectric constant insulator surrounding a HAR hole of low (i.e., air) dielectric constant acts as an "anti-waveguide" to repel the light from the contact hole. This phenomenon behaves much the same as a leaky mode in a waveguide.
There is, therefore, a need for a means that will permit the detection of defects not revealed by optical scanning according to the present art, and, in particular, the detection of the total or partial obstruction of contact holes by non-conductive matter.
Other defects of interest for the present invention are, for example, residual metal defects. These defects are of particular interest since they can short out the circuit. Such defects can show up especially in metal deposition and demacene processes, depicted in FIGS. 1B and 1C respectively. In FIG. 1B, a metal layer is deposited upon the insulator layer 100. Then, trenches (or other structures) 130 are etched, thereby leaving only metal structures 110 upon the insulator 100. However, it may occur that some metal residue 140 remains inside the trenches, and may cause a short. In FIG. 1C, trenches (or other structures) are first formed in insulator layer 105. Then, the trenches are filled with metal (generally tantalum or copper) and the entire structure is polished to remove any excess metal from the top surface of the insulator 105, thereby forming conductive structures 115 (such as bit lines). However, some scratches may form on the top surface of the insulator and be filled with the metal, thereby creating a metal residue defect 125, which can potentially short the circuit. Thus, it is important to detect the presence of such metal residue defects.
Another technology of interest for the present invention relates to charged particle energy analyzer. Specifically, a specimen is placed in the analyzer, and is bombarded with ionizing radiation so as to dislodge charged particles from the specimen's surface. The particles are then collected, and their energy spectrum is used to determine the chemical constitution of the surface of the specimen. With respect to this technology, the reader may refer to U.S. Pat. Nos. 5,185,524 and 5,286,974.
A well known technique to release charged particles, i.e., electrons from a material's surface is by illumination of the surface. This field is sometimes referred to a photo emission electron microscopy (PEEM) and has previously been used to observe samples. For an example of a PEEM for use with biological samples the reader is referred to U.S. Pat. No. 5,563,411 to Kawata et al. For use of PEEM in analysis of semiconductor structure the reader is referred to "Characterization of p--n Junctions and Surface states on Silicon Devices by Photoemission Electron Microscopy" M. Gieson et al., Appl. Phys. A 64, 432-430, 1997. More relevant to the present invention is the following discussion of the physics behind photo-emission. When the energy of a photon E=hc/.lambda. that impinges on the material is larger than its working function, i.e. .phi.&lt;hc/.lambda., there is a probability P that an electron will be released from the surface. Here, E is the kinetic energy of a photon, h is the Plank constant, and c is the speed of light.
The working function of any material represents the amount of energy that one should apply in order to release an electron from the material surface. Usually, the working functions of metals are much lower than those of insulators, because the amount of free electrons in metals is larger than in insulators. The working functions of some commonly used metals in the semiconductor industry and their corresponding wavelengths are summarized in the table below.
Metal .phi. (eV) Wavelength (nm) Al 4.28 289 Cu 4.65 266 Ti 4.33 285 W 4.55 270
As can be seen from the table, a wavelength of less than 266 nm, e.g., deep UV illumination, is required in order to release electrons from these metals.
Another technology of interest for the present invention relates to inspection of specimen using x-rays. Generally, an electron beam is caused to impinge upon a metal, such as aluminum, to generate x-rays. The emitted x-ray is formed into a beam and caused to impinge upon the specimen, thus causing emission of photoelectrons. The photoelectrons are collected and an analyzer is used to determine the chemical species on the surface of the specimen. Such technology is described, for example, in U.S. Pat. Nos. 5,444,242 and 5,602,899 both to Larson and both assigned to Physical Electronics, Inc. Physical Electronics' marketing literature (see Physical Electronics document No. 9801 authored by Dan Hook) describes the use of such technology for inspecting wafers to determine the presence of photoresist residue. However, as can be understood from the cited patents and literature, such system is cumbersome and may not be readily implemented for "in-line" inspection of wafers, especially as far as tilting the wafers is concerned. Also, the throughput is limited due to the relatively low intensity of the X-ray beam that is produced with the aid of an energetic electron beam that impinge upon an Aluminum specimen.